Wave retardation lines having periodic tapering pitch



April 2, 1963 R. c. HERGENROTHER WAVE RETARDATION LINES HAVING PERIODICTAPERING PITCH Filed Oct. 50, 1958 lNVE/VTOR HUDOLF Q HERGE/VROTHEI?ATTORNEY United States Patent 3 083,443 WAVE RETARDATIOIQ LINES HAVINGPERIODIC TAPERING PITCH Rudolf C. Hergenrother, West Newton, Mass.,assignor to Raytheon Company, Lexington, Mesa, a corporation of DelawareFiled Oct. 30, 1958, Ser. No. 770,732 2 Claims. (Cl. 29-1555) Thisinvention relates to electromagnetic wave retardation structures,commonly known as delay lines, and more particularly pertains to amethod for manufacturing a laminated delay line of the periodic type inwhich the pitch of the line progressively decreases. Delay lines havinga periodic structure are employed in beam traveling wave electron tubes,such tubes being characterized by the exchange of energy between a beamof high velocity charged particles and an electromagnetic wavepropagating along the delay line.

For a general description and discussion of the development of the beamtraveling wave tube, reference is made to Proceedings of the I.R.E. ofMarch 1956, p. 333 et seq., The 0 Type Carcinotron Tube by P. Pallueland A. K. Goldberger. A traveling wave tube utilizes an electron gunwhich generates an electron beam, the beam being directed through a longevacuated tube until it impinges upon :a collector electrode whichabsorbs the beam. Within the tube and adjacent the path of the electronbeam, there is situated a delay line for guiding and retarding thepropagation of an electromagnetic wave. The electromagnetic wave travelson the delay line at a speed approaching the speed of light but due tothe folded configuration of the delay line, the velocity of the wave inrelation to the longitudinal axis of the tube is determined by the pitchof the delay line and is a fraction of the speed of light. The electrongun and collector electrode potentials are arranged so that the averageaxial velocity of the electron beam is equal to or somewhat greater thanthe axial wave velocity. Interaction of the electron beam and the highfrequency field associated with the electromagnetic wave on the delayproduces an exchange of energy. The greater the electron current densityand the longer the delay line, the greater is the exchange of energy. Intransit through the tube, the elcetron beam becomes bunched in certainpotential regions of the traveling wave and this bunching effect resultsin a general retardation of electrons in the beam, the average electronvelocity being reduced, and the diminution of energy represented by thisdecreased velocity being imparted to the electromagnetic wave.Physically, this retardation or general lowering of the average electronvelocity is a consequence of the transfer of energy from the beam to thetraveling electromagnetic wave. If electrons in the region of couplingwith the electromagnetic wave travel at an axial velocity somewhat lessthan the axial velocity of the electromagnetic wave, then this bunchingeffect, which tends to synchronize the axial flow of electrons with thedesirable potential regions of the traveling wave, results in a generalincrease of average electron velocity. This velocity increase whichrepresents an increase in overall beam energy level is effected byextracting energy from the traveling wave and is obviously undesirablewhere the purpose is to transfer energy from the beam to the wave. Toprevent this undesirable effect, the traveling wave must beprogressively slowed as the electrons deliver their energy to the waveso that the retarded electrons remain in synchronism with the wave. Thetraveling wave can be progressively delayed by providing a periodicdelay line having its pitch tapering from one end of the line toward theother, and this invention is di- "ice rected toward a method formanufacturing such a delay line.

The invention resides in a method for manufacturing a periodicallytapering delay line constructed from laminates. The laminates arepunched from sheet stock having a uniform thickness and the stampingsare then attached to a holder. A set of laminates for a section of thedelay line would be attached to a holder and inserted in anelectroplating bath. The plating process is performed in a manner suchthat the thickness of plate deposited on the laminates tapers uniformlyover the complete set. The laminates are then held in the holder untilfinal assembly when they are removed in succession by the operator inorder to prevent the laminates from becoming mixed during the finalassembly. The assembled laminates are then placed in an oven containingan atmosphere of hydrogen and heated until the assembly is bonded intoan integral delay line section.

The method of manufacture and its advantages will be better understoodby a perusal of the following description when considered in conjunctionwith the accompanying drawings wherein:

FIGS. 1 and 2 illustrate periodic interdigital delay lines;

FIG. 3 illustrates laminates employed in the construction of a laminatedinterdigital delay line;

FIG. 4 illustrates a segment of an assembled laminated delay line; and

FIGS. 5 and 6 illustrate apparatus for electroplating the laminateswhich are subsequently assembled to form an interdi-gital delay line.

The method of manufacture relates in general to delay lines which can beconstructed from laminates. For purposes of discussion, however, thedescription will be limited to interdigital delay lines of the typeshown in FIG. 1 and the generality of the method may be deduced from thedescription herein. The delay line shown in FIG. 1 may be viewed ascomprising two combs 1 and 2 having their fingers or teeth 3 interleavedand fixed in relation to one another. The pitch P of the line is definedas the distance between corresponding points on adjacent fingersprojecting from the same comb. Electromagnetic wave energy is consideredto propagate in the space bounded by the two combs. Assuming a constantvelocity of propagation in that space it may be seen that by graduallydecreasing the pitch of the line, the propagation path of theelectromagnetic wave energy will be lengthened thereby causing the waveto be slowed down as it progresses along the delay line. The effect ofreducing the pitch on the velocity of wave propagation can be more fullyappreciated by comparing FIG. 1 with FIG. 2 which shows 'a section of aninterdigital delay line equal in length to the delay line of FIG. 1 buthaving a smaller pitch P2 and,

hence, more fingers. An electromagnetic wave propagatrug from the pointa to the point b in FIG. 1 is constrained to follow the path indicatedby the broken line 4 and will traverse that path in a time 1. Anelectromagnetic wave starting from the point e in FIG. 2 will travel anequal distance in the time 1 along the broken line path 4a and willtherefore reach the point 1 at the expiration of time 1. Whereas in FIG.1 the electromagnetic Wave has moved a distance al along the delay linein the time 2, the electromagnetic wave in FIG. 2 has in the same timemoved a lesser distance d along the delay line because of its smallerpitch. It is clear, therefore, that a decrease in pitch of the delayline cause the wave to progress longitudinally along the line at aslower speed.

FIG. 3 illustrates three types of stampings or laminates which areemployed to form an interdigital delay line. The laminates 5 and 6 arecharacterized by dependent fingers 7 and 8- which protrude in oppositedirections. The laminate 9 is a spacer element which is interposedbetween the laminates 5 and 6 and therefore determines the spao ingbetween fingers 7 and 8. By assembling the laminates in the sequenceshown in FIG. 3, a delay line of any desired length may be formed. -Whenthe laminates are assembled the apertures 11, 12 and 13 therein arealigned and form a conduit through which a co-axial line centerconductor may be inserted to form an output coupling. A unitary delayline is made of a large number of thin laminates brazed together to forman integrated laminar structure. The laminates shown in FIG. 3 arepunched from sheets of copper or an alloy of copper. The metalemployed-in the stampings should be ductile, free of oxygen, and havehigh electrical conductivity. A variety of copper known as OFHC copperhas been employed with satisfactory results. Laminates 5 and 6 areusually stamped from sheet stock of the same uniform thicknes whereasthe spacer 9 is stamped from sheet stock having a gage equal to thedesired spacing between adjacent fingers. It should be understood that aconventional interdigital delay line may have 100 or more fingers andthat delay lines having 200 fingers are not uncommon. The number offingers in a delay line does not impose any limitation upon the methodof manufacture described herein, as will be appreciated from a completereading of this description. The dies for making the stampings are veryaccurately constructed and are hardened so that large numbers ofstampings may be struck from thin sheets of copper while maintaininghigh dimensional precision. The stampings or laminates, after beingformed by the dies, are cleaned, for example, by 'anodic pickling, torender their surfaces free from grease and other contaminants. As aprecaution, the laminates should not be handled with. bare hands untilat'ter firing in a brazing furnace because the skin secretes oils andinorganic salts which interfere with the formation of a good bond. Thestresses set up in the copper stampings by the die-stamping operationarerelieved by anneal ing the copper while concurrently applying a uniformpressure to prevent curling or bending of the stamping. The annealingoperation is preferably performed in a reducing atmosphere to remove anyoxides which may have formed on the stampings.

Assuming the requisite number of laminates of each type have beenstamped from sheet copper, cleaned, and

annealed, laminates of the type having dependent fingers.

are then arranged along a rod so that a laminate-having a downwardlyprotruding finger alternates with a laminate having an upwardlyprotruding finger. The number of laminates spaced along the rod dependsupon the number of fingers which it is desired to incorporate'in thecomplete delay line or a delay line section. That is, a delay line maybe constructed in laminar sections which are subsequently bondedtogether to form the complete delay line or, if the delay line is ofmoderate length, the complete laminated delay line may be fabricated asa unit. The total length of the complete delay line will govern theprocedure best suited to the individual case. The rod 14 is fabricatedof an insulative material and has attached to it a number of holders orclamps 15 from which the laminates are suspended. The rod and thesuspended laminates are then placed in a copper plating bath and thelaminates are electroplated by applying an electric potential betweenthe laminates and a suitable oppositely polarized electrode. In orderthat the thickness of copper plate deposited on the laminates shallprogressively decrease from one end of the rod to the other, two methodsof arrangement are here described which achieve the desired results. Thefirst method is illustrated in FIG. 5 which shows a tank 16 containing acopper plating solution in which are immersed the laminates 17, 18, 19,2.0

suspended from the insulative rod 14 by suitable holders.

A constant speed motor 21 is provided which drives a shaft 22 carrying anumber of discs which rotate with the shaft. To the peripheral surfaceof each disc, the disc 23 for example, there is secured a conductivestrip 24 and a radial Conductive lead 25 provides an electrical 4 pathfrom the strip to the metallic shaft. A brush 26 is positioned to bearon the periphery of disc 23 and an electrical connection 27 is providedbetween brush Z6 and the laminate 17. In a similar manner, each of theother laminates in the bath is electrically connected to a brush bearingon the surface of a difierent disc. The conductive strip-s (exemplifiedby the strip 24) secured to the discs are of unequal lengths and arearranged so that the longest laminate.

strip is secured to the disc 23 at one end of the shaft and eachsuccessive disc carries a strip of smaller length. A source ofelectrical current, here indicated by the battery 23, is connectedbetween shaft 22 and an electrode 29 immersed in the tank. As shaft 22turns slowly through a complete revolution, the duration of current flowbetween each laminate and the electrode 29 is determined by the lengthof the conductive strip in the periphery of its associated disc. Theduration of current flow, in turn, determines the thickness of copperplate deposited on the laminate. Thus, in FIG. 5, the laminate 17 willhave the thickest copper plate and each successive laminate will have athinner copper plate.

The second arrangement for insuring that the copper plate deposited onthe laminates shall progressively decrease from one end of the rod tothe other is illustrated in FIG. 6. In this latter arrangement thelaminates, suspended from :a rod in the manner previously described,

are immersed in a copper electroplating bath. The electrode 30 isdisposed at an angle to the suspended lamimites and an electricpotential is impressed for a predetermined length of time between thelaminates and the electrode 30 by a source, here illustrated as abattery 31. The fiow of current between each of the immersed lamimitesand the electrode is directly related to the spacing between those"members and hence, the thickness of copper plate deposited on alaminate varies inversely with the distance between it and the electrode30. That is, the larger the current, the more copper is deposited on :1Thus, for the same period of current flow, the greatest thickness ofplate will be deposited on laminate 32 with each consecutive laminatehaving a plating of lesser thickness. When the desired thicknesses ofplate have been deposited, the current is turned off and the rod withits suspended laminates are removed from the bath.

Regardless of the process used to plate the laminates, the laminates areretained on the rod 14- until the final assembly stage is reached.indiscriminate mixing of the plated laminates must be avoided. This canbe more fully appreciated when it is realized that the difference inthickness between adjacent laminates may be of the order of one onehundred thousandth (l/100,000) of an inch. At the final assembly stage,the laminates are assembled on a fixture having a means for preciselyaligning the dependent fingers. In the process of assembly, the platedlaminates are successively removed from the rod beginning at one end andthe laminates having dependent fingers are alternated with a spacer sothat the laminated stack consists of laminates such as 5 and 6 in FIG. 3and interleaved spacers such as the spacer 9. The assembled stack oflaminates is placed in a rigid frame and placed in a brazing furnacewhere the laminated stack is subjected to compressive pressure while thetemperature is raised to brazing heat.

.A suitable brazing procedure isknown in which the spacers, i.e., thoselaminates not having a dependent finger, are plated with a coat ofsilver having a thickness of the order of .0002". The laminates areassembled in the manner previously indicated, the stacked laminates areplaced in a steel frame having /s the expansion of copper, and theassembly is then placed in a brazing furnace. Upon heating the furnaceto brazing temperature, i.e. the temperature at which the silver-coppereutectic melts, high intensity pressures are induced in the laminatedstack because of the difference in the rates of thermal expansionbetween the copper laminates and the steel frame. The laminated stack issubsequently slowly cooled so that the copper is again annealed. Thisprocedure has proved to be highly successful in the formation of avacuum tight bond without an accompanying excess of silver solder. For afuller description of this brazing method, reference may be made tocopending application Serial No. 627,453, filed December 10, 1956, nowPatent No. 2,882,587.

FIG. 4 shows a cross-sectional view through a segment of an assembleddelay line. It will be observed that the thickness of the fingersgradually diminishes as one progresses from the left end toward theright. The change in thickness has been greatly exaggerated in thedrawings since in an actual delay line the diiference in thickness isnot readily detected by the unaided eye. It will also be noted that thespacers 34, 35, 36, 37 are of the same thickness so that the spacingbetween fingers is maintained uniform throughout the delay line.However, where it is desired to change the impedance along the delayline in a gradual manner, the spacers 34, 35, 36 and 37 may be plated inthe same manner as the laminates having fingers so that the spacingbetween fingers is not uniform but tapers from one end of the line tothe other.

While the invention has been described with reference to copperlaminates, the method here disclosed is applicable to many copper alloysand to other metals such as nickel and related alloys such as Monel.This invention, therefore, is not limited to the particular details ofconstruction or materials described. It is accordingly desired that theappended claims be given an interpretation commensurate with the scopeof the invention.

What -I claim is:

1. A method of constructing a laminar delay line comprising the steps offorming metal spaces, punching metallic laminates having dependentdigits from sheet stock having a uniform thickness, suspending saidlaminates from a holder in the order in which said laminates are to beassembled, immersing said laminates in an electroplating bath, causingeach of said laminates to be plated for a different length of timedetermined by its position upon said holder, removing said holder andits suspended laminates from said bath, consecutively removing saidlaminates starting at one end of said holder, alternately stackingspacers and said laminates, and bonding the stack into an integral unit.

2. A method of constructing a laminar delay line comprising the steps offorming metal spacers, striking laminates having dependent digits fromsheet metal of uniform thickness, arranging said laminates upon a holderin the order in which said laminates are to be assembled, immersing saidlaminates in an electroplating bath having an electrode diverging fromsaid holder, concurrently connecting said laminates in parallel to asource of electric current whereby each of said laminates are coatedwith a metallic plate Whose thickness is determined by the distancebetween the laminate and said electrode, removing said holder and theattached laminates from said bath, alternately stacking said spacers andlaminates upon a jig which aligns said dependent digits, andsimultaneously applying pressure and heat to the stacked assemblage toform an integral structure.

References Cited in the file of this patent UNITED STATES PATENTS732,616 Burgess et al. June 30, 1903 1,529,249 Gue Mar. 10, 19251,712,284 Turnock May 7, 1929 2,149,344 Hull Mar. 7, 1939 2,456,457Somerville Dec. 14, 1948 2,641,731 Lines June 9, 1953 2,724,691 HakesNov. 22, 1955 2,882,587 Unger et al. Apr. 21, 1959

1. A METHOD OF CONSTRUCTING A LAMINAR DELAY LINE COMPRISING THE STEPS OFFORMING METAL SPACES, PUNCHING METALLIC LAMINATES HAVING DEPENDENTDIGITS FROM SHEET STOCK HAVING A UNIFORM THICKNESS, SUSPENDING SAIDLAMINATES FROM A HOLDER IN THE ORDER IN WHICH SAID LAMINATES ARE TO BEASSEMBLED, IMMERSING SAID LAMINATES IN AN ELECTROPLATING BATH, CAUSINGEACH OF SAID LAMINATES TO BE PLATED FOR A DIFFERENT LENGTH OF TIMEDETERMINED BY ITS POSITION UPON SAID HOLDER, REMOVING SAID HOLDER ANDITS SUSPENDED LAMINATES FROM SAID BATH, CONSECUTIVELY REMOVING SAIDLAMINATES STARTING AT ONE END OF SAID HOLDER, ALTERNATELY STACKINGSPACERS AND SAID LAMINATES, AND BONDING THE STACK INTO AN INTEGRAL UNIT.2. A METHOD OF CONSTRUCTING A LAMINAR DELAY LINE COMPRISING THE STEPS OFFORMING METAL SPACERS, STRIKING LAMINATES HAVING DEPENDENT DIGITS FROMSHEET METAL OF UNIFORM THICKNESS, ARRANGING SAID LAMINATES UPON A HOLDERIN THE ORDER IN WHICH SAID LAMINATES ARE TO BE ASSEMBLED, IMMERSING SAIDLAMINATES IN AN ELECTROPLATING BATH HAVING AN ELECTRODE DIVERGING FROMSAID HOLDER, CONCURRENTLY CONNECTING SAID LAMINATES IN PARALLEL TO ASOURCE OF ELECTRIC CURRENT WHEREBY EACH OF SAID LAMINATES ARE COATEDWITH A METALLIC PLATE WHOSE THICKNESS IS DETERMINED BY THE DISTANCEBETWEEN THE LAMINATE AND SAID ELECTRODE, REMOVING SAID HOLDER AND THEATTACHED LAMINATES FROM SAID BATH, ALTERNATELY STACKING SAID SPACERS ANDLAMINATES UPON A JIG WHICH ALIGNS SAID DEPENDENT DIGITS, ANDSIMULTANEOUSLY APPLYING PRESSURE AND HEAT TO THE STACKED ASSEMBLAGE TOFORM AN INTEGRAL STRUCTURE.